Magnetic hard disk drives include a rotating rigid storage disk and a transducer positioner for positioning a read/write transducer at different radial locations relative to the axis of disk rotation, thereby defining numerous concentric data storage tracks on each recording surface of the disk. The transducer positioner is typically referred to as an actuator. Although numerous actuator structures are known in the art, in-line rotary voice coil actuators are now most frequently employed due to their simplicity, high performance, and mass balance about their axes of rotation, the latter being important for making the actuator less sensitive to perturbations. The in-line rotary voice coil actuator is less susceptible to disturbances external to the disk drive, which can otherwise move the read/write transducer to an unexpected position over the storage disk. A closed-loop servo system within the disk drive is conventionally employed to operate the voice coil actuator and position the read/write transducer with respect to the disk storage surface.
The read/write transducer, which may be of a single or dual element design, is typically mounted on a ceramic slider structure, the slider structure having an air bearing surface for supporting the read/write transducer at a small distance away from the rotating storage disk. Single read/write transducer designs typically require two-wire connections while dual designs having separate reader and writer elements require a pair of two-wire connections.
Sliders are generally mounted on a gimbaled flexure portion. The gimbaled flexure portion is attached to one end of a load beam assembly. An opposite end of the loadbeam assembly is attached to the in-line rotary voice coil actuator, which provides pivotal motion to the suspension assembly. A spring biases the load beam and the slider with the read/write transducer towards the disk, while the air pressure beneath the slider developed by disk rotation relative to the slider pushes the slider away from the disk. The gimbaled flexure enables the slider to present a "flying" altitude toward the disk surface and to follow its topology. An equilibrium distance defines an "air bearing" and determines the "flying height" of the read/write transducer. Although the separation between the read/write transducer and disk created by the air bearing reduces read/write transducer efficiency, the avoidance of direct contact of the transducer with the disk vastly improves reliability and extends the useful life of the read/write transducer and disk. The air bearing slider and read/write transducer combination is also known as a read/write head/slider assembly (herein "head").
Currently, nominal flying heights are on the order of 0.5 to 2 microinches. For a given read/write transducer, the magnetic storage density increases as the read/write transducer approaches the storage surface of the disk. Thus, a very low flying height is traded against transducer reliability over a reasonable service life of the disk drive. As a corollary, maintaining the same predetermined flying height over each disk incorporated into a disk drive and between different disk drives of the same type is critical.
One problem arises as a result of suspension assemblies that have variations in intrinsic stiffness. Variations in the intrinsic stiffness of each of the suspension assemblies incorporated into a multi-platter disk drive result in heads that fly at different heights. Because the efficiency of the magnetic recording process changes significantly with the flying height, variations in flying height result in corresponding variations in recording storage densities. Because it is practically impossible to know the variations in flying height of any given head/suspension combination, multi-platter disk drives typically record data at less than optimal storage densities to allow for the variations in flying heights of the heads incorporated therein. This lowers the overall storage capacity of the multi-platter disk drive.
An additional problem in multi-platter disk drives results from variations in Z-heights between each suspension assembly and its respective storage surface. The Z-height of a suspension assembly is defined as the vertical distance between the baseplate of the suspension assembly and the storage surface of its associated storage disk. Variations in Z-heights result from the numerous deviations from predetermined dimensions that occur during the manufacturing and assembly process of the disk drive. In particular, the staking process used to attach the baseplate of the suspension assembly to the head arm of the rotary actuator minutely deforms a portion of the suspension assembly. These minute deformations of each suspension assembly affect the Z-height of each suspension assembly within the drive. The variations in Z-heights of the suspension assemblies are further translated to each head attached thereto. Other areas where manufacturing variances can result in deviations from predetermined dimensions include the spacing between the actuator arms, spacings between the storage disks formed by storage disk rings or disk spacers, and general positioning of components and torque settings of fasteners within the disk drive.
A conventional method of compensating for variations in both suspension assembly intrinsic stiffness and suspension assembly Z-height is to adjust the stiffness and Z-height of the suspension assembly during the manufacturing and assembly process of the disk drive. Typically these variations are adjusted by locally heating a portion of each suspension assembly with a laser. This localized heating changes the spring constant thereof. As a result, the spring constant of each suspension assembly is adjusted to have a predetermined gramload force so that all the heads employed within a single or multi-platter disk drive will fly at a substantially uniform flying height.
One drawback to using a laser beam to provide localized heating of the suspension assembly is that heating must be completed prior to final assembly and operation of the disk drive. The gramload of a suspension assembly and the resultant flying height of the head attached thereto are typically dictated by estimated defects in the flatness of the rotating storage disk employed within the disk drive. Typical defects in the flatness of storage disks are known as asperities. An asperity is generally a small bump on the storage surface of the storage disk that is formed during the manufacturing process of the disk. Because asperities have varying heights, those having a height greater than the fly height may be contacted by the head, causing the temperature of the head to rise abruptly. Such asperities are known as thermal asperities. If a head repeatedly hits thermal asperities, the head can be damaged or its life expectancy may be reduced. Therefore, a disk drive designer must estimate the approximate height of all potential asperities and adjust the gramload of each suspension in the disk drive so that the attached heads will fly over the highest predicted asperity.
Because the dimensions and positions of asperities are fundamentally unpredictable, disk drive designers have had to adjust suspension assembly gramloads to fly heads at conservative minimum flying heights to insure adequate clearance between the heads and the asperities. However, heads that fly at these predetermined conservative heights over disks that don't have asperities unnecessarily and severely reduce the storage capacity of the disk drive that they are incorporated into.
Thus, a hitherto unsolved need remains for a method of and apparatus for adjusting the gramload of a suspension assembly so that the head will fly at the minimum height possible without subjecting the head to thermal asperities, thus optimizing the life expectancy of the head and the storage capacity of a disk drive simultaneously.